Note: Descriptions are shown in the official language in which they were submitted.
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WIRE COATING COMPOSITION
The present invention relates to a wire coating
composition. The term ~'wire" used herein embraces both wires
and cables.
It is conventional to provide a polymeric insulation
coating around wires and cables. Such coatings are often made
from halogenated polymers, such as polyvinyl chloride (PVC).
The main problem of PVC and other halogenated polymers is that
upon combustion a large volume of toxic, acidic and highly
corrosive hydrogen halide smoke is liberated. As a
consequence halogen-free polymer compositions for coating
cables or wires have been developed. Examples of such
compositions are included in EP 082407A, EP 488381A and US
5032321. These and other prior art halogen-free coating
compositions, which are based upon thermoplastic
polymer/mineral filler mixtures, possess inadequate abrasion
resistance, electrical insulation properties and temperature
stability. These coating compositions have a maximum
temperature range of only 70-80~C. Furthermore, the extrusion
speed for compositions of the type disclosed in EP 082407A,
EP 488381A and US 5032321 may only be one fifth of that for
PVC. Similar compositions containing intumescent system flame
retardants, such as ammonium polyphosphate, have also been
shown to exhibit inadequate electrical resistance due to the
fact that the flame retardant attracts moisture and therefore
increases the electrical conductivity of the material. It is
noted that conventional phosphorous-based flame retardant
compounds are contained in relatively large amounts, usually
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up to 40 php for ammonium polyphosphate.
A disadvantage with red phosphorous-based system is that
the cable coatings have a strong red colouring, which is
disadvantageous for electrical cable coatings in general.
The aim of the invention is to provide a recyclable
and/or reprocessable halogen-free coating composition with
improved abrasion resistance, electrical insulation and
temperature resistance properties without a deterioration in
other physical properties, such as tensile strength or
flexibility.
According to the present invention there is provided a
wire coating composition comprising polypropylene polymer or
copolymer, polyethylene wax and magnesium hydroxide provided
with a hydrophobic coating and wherein the wire coating
composition does not contain any halogen and is essentially
free of phosphorous and phosphorous compounds and phosphorous
salts.
The term "essentially" is used herein to mean that the
composition is free from "phosphorous" or compounds or salts
thereof or has a phosphorous content of less than 800 ppm.
The wire coating composition of the present invention
exhibits the required coating characteristics, but unlike some
PVC compounds is readily recyclable. The coating composition
of the invention further provides the required electrical
insulation, while being flexible, flame resistant, heat stable t
to greater than 125~C, abrasion resistant, readily extrudable
and recyclable.
The components of the composition are ideally present in
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the following ratio:
polypropylene 100 php (parts per hundred
polypropylene);
magnesium hydroxide 100 to 200 and ideally
Y (coated) substantially 140 php; and
polyethylene 1 to 20 and ideally substantially
10 php.
The coating compositions are particularly useful for
wires in motor vehicle engines, such wires being subjected to
high temperatures and contact with water and fluids such as
petrol, diesel, oil, salt solution and anti-freeze. It is
necessary for wire coating materials under vehicle bonnets to
have a temperature rating of greater than 100~C, at which
temperature the coating should remain stable and retain
electrical insulation properties. The coating compositions
according to the invention possess a temperature rating of
greater than 125~C.
Polyethylene wax is incorporated to improve abrasion
resistance. Hard high molecular wax components would provide
excellent abrasion resistance, but would also increase the
tendency for stress cracking of the coating to occur. Being
a relatively low molecular weight material, polyethylene wax
enhances abrasion resistance of the coating without having an
adverse effect on other coating properties.
The magnesium hydroxide acts as a filler and flame
retardant. In order to overcome the problems of moisture
attraction associated with intumescent flame retardant
systems, the magnesium hydroxide particles are coated with a
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hydrophobic material such as an alkyl silane, for example a
trimethoxysilane or triethoxysilane. The coating enhances
adhesion between the filler particles and the polymer matrix,
improves the abrasion resistance of the coating and most
importantly, due to its hydrophobic nature, increases the
resistance of the coating to moisture entrapment thereby
maintaining electrical insulation properties. Magnesium
hydroxide is also stable at high temperatures, whilst its low
surface area gives it a low viscosity. It is therefore easy
to process.
The preferred amount of 140 php magnesium hydroxide
content gives the ideal balance between flame retardance and
flexibility. A high filler content gives excellent flame
retardance. However, at the same time the coating will become
much less flexible such that the coated wire will fail a
standard conductance test whereby a wire is tightly coiled
(typically around a lmm diameter mandrel) and stored in hot
water (normally 80~C) whilst under high voltage. If the
coating is not flexible enough then it will crack at the coil
heads.
A further advantage of the compositions according to the
invention is that they can be extruded onto wire or cable
using existing extrusion equipment at speeds similar to that
of PVC coatings.
A cost saving may be made by using the coating
compositions of the invention using the high temperature
rating of the coatings. Normally a copper wire becomes hot
due to the conduction of the electrical current. The high
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W096/2788S PCT/GB96/00501
temperature rating of the coating means that thin layers of
coating can be used on thin wires, representing a saving of
expensive copper.
A preferred embodiment of the invention involves the
i incorporation into the composition of a synergistic blend of
an anti-oxidant derived from phenol and anti-oxidant based
upon a phosphite. Such anti-oxidants are present in an amount
of only 0.1 to O.S php. Such phosphite contents are
sufficiently low (770 ppm) to have a negligible moisture
attraction capability. In such compositions good
processability is retained without adversely effecting the
other physical properties of the coating. This aids further
reprocessing of the coatings and therefore improves the
recyclability of the compositions.
The polymeric composition of the invention may optionally
further comprise additives such as stabilisers, plasticisers,
lubricants and further flame retardants.
In order that the invention may be more readily
understood specific embodiments of the invention is now
described in detail.
Example 1
Composition: 100 php propylene polymer1
140 php Magnifin H5KV2 coated with AMEo-T3
5 php polyethylene wax4
1 polymer comprises 20 php Appryl 3060 MMS (tradename of
Atochem for a propylene copolymer) and 80 php Moplen D50-G
(tradename of Himont for a propylene homopolymer)~ Appryl
3060 MMS has a melt flow index of 0.5g per minute at 2300C and
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21.6 kg. Moplen D50-G has a melt flow index of 0.3g per
minute at 230~C and 21.6 kg.
2 magnesium hydroxide - a tradename of Martinswerk.
3 octyl triethoxy silane coupling agent, a tradename of
HUls.
4 The trade name of the wax is A-C 9/9A (tradename of
Allied Wax). This has a drop point of 115~C and a density of
0.93 g/cm3.
The composition was extruded onto a standard metal wire
used for automotive electrical cable applications, at a rate
of about 700 m per minute, using the normal extrusion
equipment for PVC-coated cables, to form a coated cable with
a cross-sectional area of 2.5mm2 Such cables would commonly
be used as alternator or battery leads in a motor vehicle
englne .
The test results for the coating are as follows:-
Requirement St~n~rd Resuits
Insulation Conductive cross-sectional DIN 72551 39 N
area of 1.5 - 2.5 mm2 under Point 3.3.2
a load 10 - 80 N
Abrasion Behaviour measured 1500 cycles DIN 72551 ~ 3000
in terms of the number of hub Point 3.3.3
cycles to retain a min. thickness of
2.5 mm2
Heat Resi;,lclnce Under Load 0.15 mm DIN 72551 ~ 0.25 mm(measured as the pen~, c,tion Point 3.4.2
depth into the coating after heating
for 4 hours at 125~C
Max. Thermal Shrinkage 4 % DIN 72551 2.5 % Point 3.4.2
Voltage Strength for 30 minutes 5 KV DIN 72551 passed
Point 3.5.3
Min. Specific Resi:,ldnce at 70~C 1 o10 ohms per cm DIN 72551 3.2 x 1014
Point 3.6.1
Thermal Shock (150~C for 48 hrs) Voltage Strength 1 KV DIN 72551 passed
Point 3.6.2
Cold Strength ( - 40~C) Voltage Strength 1 KV DIN 72551 passed
Point 3.6.3
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Storage In M ' Re~;on Engine Voltage Strength 1 KV DIN 72551 passed
Oil (SAE 10 W-50) for 24 hrs at Point 3.6.2
90~C
Storage In Brake Fluid Voltage Strength 1 KV DIN 72551 passed
(Teyes ATE DOT 3) for 24 hrs at Point 3.6.2
~1 sooc
Storage In Four Star Petrol Voltage Strength 1 KV DIN 72551 passed
(see DIN 51601) for 24 hrs at Point 3.6.2
room temperature
Storage In Diesel Fuel Voltage Strength 1 KV DIN 72551 passed
(see DIN 51601) for 24 hrs at Point 3.6.2
room temperature
Example 2
Composition: lO0 php propylene polymer1
140 php Magnifin H5KV coated with
AMEO-T
5 php polyethylene wax
l polymer comprises 40 php Appryl 3060 MMS and 60 php
Moplen D50-9.
The composition of Example 2 also passed the relevant
tests which were carried out in relation to the composition
of Example l.
It is to be understood that the embodiments described
above were by way of illustration only. Many modifications
and variations are possible.
